Timing of breeding, clutch size and double-broodedness in Barn Swallows Hirundo rustica

Abstract

In a population of Barn Swallows (Hirundo rustica) in central Poland studied in 1994-1997, the mean onset of egg laying ranged from 18 to 30 May and mean clutch sizes were 4.86 ± 1.02 (SD) for first clutches, 4.47 ± 0.75 (SD) for second clutches and 4.26 ± 0.98 (SD) for repeat clutches. Clutch sizes were negatively correlated with laying dates and varied between the brood categories, i.e. first, second and repeat broods, but not between years. The frequencies of single-brooded and double-brooded females differed between years. Yearling or immigrant females started breeding later, produced smaller clutches and tended to be single-brooded more often than older ones. Consequently, multi-broodedness may be considered to be an age-related characteristic in Barn Swallows. The seasonal decline in clutch size may also be, at least in part, explained by a relation to female age. The observation for Barn Swallows that clutch sizes consistently decrease during the course of the breeding season does not fit the hypothesis for multi-brooded bird species. In this hypothesis, clutch sizes initially increase with the date of laying and only after reaching a peak do they decrease.